Michael Faraday was a scientist of the 1800s. His work contributed to the field of electromagnetism, among other scientific principles. Important to the present disclosure, Faraday proposed the law of electromagnetic induction, based upon his observations of current induced in a conductor when exposed to magnetic fields. Simply stated, Michael Faraday's proposition of electromagnetic induction was that exposure to a change in magnetic flux produces current in the conductor.
In Faraday's work, he created a homopolar generator, which is a direct current (DC) electrical generator. The homopolar generator consisted of an electrically conductive disc that rotated in a plane perpendicular to a static magnetic field. The rotation in the magnetic field created a current flow in the conductive disc.
Based upon Faraday's research, the current produced was a result of the conductive disc breaking the lines of magnetic flux emanating from the magnet. However, Faraday found that when both the conductive disc and the magnet were coupled and rotated together, current was still induced in the conductive disc. This has been historically referred to as the “Faraday Paradox.” Thus, there are situations in which Faraday's law of electromagnetic induction does not appear to predict accurate results.
The question then becomes does external magnetic flux rotate or remain axially fixed in the homopolar generator. Faraday's opinion in 1831 was that the flux remained fixed as the disk and magnet rotated; however, today academia is still undecided.